Gene fusions can be used for basic studies of gene expression, structure, and function. Our purpose is to develop and improve methods for fusing genes in the bacterium E. coli, and to use these and other general methods to study in particular the genetic system of DNA transposition. These methods involve the fusion of transcription and/or translation gene control signals from genes under investigation to the structural part of a well-characterized gene such as the lactose operon lacZ gene for B-galactosidase. With these fusions the biochemical and genetic procedures available for a gene such as lacZ can be used to study other genes. Fusion methods involve both in vitro cloning vectors and in vivo bacteriophage Mu transposable elements containing various segments of the B-galactosidase gene. The cloning vectors are designed with technical improvements including many restriction sites, more types of gene segments, smaller sizes, and various replicons for different applications. The Mu elements will be designed for use in the chromosome, in plasmids, and with replicons inside the Mu element for autogenous replication. Mu fusion elements will also be designed for promoting transcription, with and without translation, for expression of adjacent structural genes. Studies of transposition involve the Tn3 ampicillin resistance DNA transposon and the bacteriophage Mu transposon. Tn3 studies are divided into studies fo the trpA transposase and its site of action for transposition, the trpR repressor-resolvase and its tri-partite binding site, the phenomena of transposition immunity, and an investigation of a fourth gene on Tn3. Mu studies involved identification of the genes and sites essential for high frequency replication and transposition, particularly as they pertain to Mu gene fusion elements.